The present invention relates generally to magnetic resonance (MR) imaging systems and, more particularly, to an RF receiver assembly capable of translating multiple channels of MR signals across a single readout cable. The present invention is particularly applicable with multi-coil architectures.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received by a receive coil(s) and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
A radio frequency (RF) coil assembly having one or more receive coils is used to sample the “echo” induced by application of magnetic field gradients and excitation pulses. Each receive coil samples the echo or MR signal and transmits the signal via an RF transmitter to an RF receiver. Each receive channel of the RF receiver then translates the acquired signal to a processing system that formats the signal into a data stream that is fed to a data acquisition system (DAS) for image reconstruction.
RF coils are generally connected to the RF transmitter and/or the RF receiver of the magnetic resonance system using coaxial cable. Coaxial cable is designed to protect the system from picking up extraneous RF signals which are present in the environment. Coaxial cables feature a surrounding shield or ground conductor separated from a current carrying central conductor by a dielectric material. The surrounding ground conductor acts as a shield that minimizes the pick-up of foreign frequencies by the central conductor of the cable.
Although coaxial cable is used, there are still coupling issues at resonance frequencies, such as 63 MHz for hydrogen dipoles in a 1.5 T B0 field. Among other things, the shield conductor of the coaxial cable itself tends to carry foreign induced currents, such as from TV transmissions, stray harmonics from the gradient pulse oscillators and clocking circuits in nearby equipment, and the like. The induced current is often referred to as “skin current” because it flows on the outside of the shield conductor. The stray RF current tends to flow out of the bore and into other circuits, such as the amplifiers, analog-to-digital converters, receivers, and reconstruction processor to contribute errors in the resultant image.
Balance/unbalance (“balun”) circuitry, common mode chokes, and/or cable traps are typically used for reducing, or “trapping,” the noise and/or stray RF currents generated due to induced currents in the coaxial cable. Baluns typically comprise an LC frequency filter for each cable located in a copper shielded box. Since baluns are resonators, they must be manually tuned to the frequency of interest to filter out or prevent the induced currents from disturbing the delicate data measurements.
The use of baluns presents several disadvantages. For example, the manual tuning process for the baluns is typically time consuming and operator intensive, thus increasing the cost and complexity of the imaging system. Also, baluns generate a significant amount of heat. In addition, baluns are useful only over a narrow frequency range. While this narrow range may be sufficient for some imaging techniques, this inherent limitation of current baluns may prove problematic for use with MNS and multi-nuclear imaging and coil design.
It would therefore be desirable to have a system and apparatus that overcomes the aforementioned drawbacks of baluns by filtering or preventing induced or stray RF currents without a balun.